The present invention relates to a process for obtaining a switching command in a rapid changeover device for switching over a consumer installation, as needed, from a first power supply source to a second power supply source. After the consumer installation is disconnected from the one power source, a differential voltage is formed in the consumer installation from the voltage of the second power supply source and from the voltage at the consumer installation. The switching command is then given, in view of the inherent response time of the power switch effecting the changeover, when a differential voltage is pre-calculated for the moment of changeover with a value below a permissible maximum value.
A process of this type can be inferred from the German Patent Publication No. 21 08 307 C3 (hereinafter "the 307 publication"). In a process such as the one inferred from the '307 publication, an auxiliary voltage proportional to the mean differential voltage is formed from the voltage of the power supply source to be connected at any one time and from the voltage at the consumer installation. Additional auxiliary voltages are formed from this auxiliary voltage through repeated differentiation, so that the auxiliary voltages represent single terms of a series expansion, preferably of a Taylor series. The auxiliary voltages are generated proportionally to the product from the derivative of the particular order and to a quantity of a corresponding power, the quantity representing a selectable time factor. The thus formed auxiliary voltages are added and compared to a specified voltage to obtain a switching command. If the selectable time factor corresponds to the inherent response time of the switch effecting the changeover, then the known process allows a changeover to be produced in view of the inherent response time of the particular switch, at an instant when the sum of the auxiliary voltages falls below a specified value, so that the changeover is substantially bumpless (i.e., seamless).
Rapid changeover devices safeguard the power supply of important consumer installations, particularly for motor configurations. Rapid changeover devices are essential for auxiliary switchgear used in power stations, for which at least two power supply sources are almost always provided, the power supply sources being as independent as possible from one another. In such auxiliary switchgear, all machines and units required for the operation are supplied from a station-service bus of a generator block. During start-up of the generator block, the required power is obtained from the power supply system. After the start-up operation is complete, the internal power consumption requirement is obtained from the block itself.
In the case of a malfunction (for example a short-circuit, a drop in frequency or voltage caused by an overload, etc.) in the supply system or the generator, a secondary power supply source must be switched to within a few seconds, to continue to safeguard the internal power consumption of the power station. During such a changeover operation, the switch to the connected power supply source is first opened, thereby disconnecting the consumer installation from the incoming power supply. As a result, the voltage at the consumer installation decreases with respect to frequency and amount with a time constant specific to the installation. After the consumer installation is disconnected from the one power supply source, the changeover to the secondary power supply source must be made as quickly as possible. However, this can only happen when the differential voltage between the secondary power supply source to be connected to the system and the consumer installation lies below an adjustable value. In this case, the inherent response time of the power switch effecting the changeover at any one time must be accounted for.
Starting from the known measuring process described above, the object of the present invention is to provide a measuring process for obtaining a switching command in a rapid changeover device, which will allow the differential voltage to be calculated with relative accuracy, in view of the inherent response time of the power switch effecting the changeover at any one time, so that the changeover from the first power supply source to the secondary power supply source will be bumpless (i.e., seamless) to the greatest possible extent.